System and method for chemical analysis using laser ablation

ABSTRACT

A system and method for chemically analyzing single particles in a high velocity gas flow. The system comprises an ion source chamber having a gas inlet and outlet, and a high-energy, pulsed, ultraviolet laser for ablating the single particles in the high velocity gas flow entering the ion source chamber through the gas inlet to produce positively- and negatively-charged ions. The system further includes a first extraction plate for extracting the positively-charged ions provided in the ion source chamber, and a second extraction plate for extracting the negatively-charged ions provided in the ion source chamber. The positively-charged ions are injected into a first ion mobility spectrometer where they are detected and characterized. The negatively-charged ions are injected into a second ion mobility spectrometer where they are detected and characterized. The dual ion mobility spectrometers configuration of the present invention permits characterization of both the positively- and negatively-charged ions from a single gas particle.

CLAIM FOR PRIORITY AND GOVERNMENT RIGHTS

[0001] The present application claims the benefit of InternationalApplication No. PCT/US01/18468, filed Jun. 7, 2001, which is based onU.S. Provisional Patent Application Serial No. 60/210,610, filed Jun. 9,2000. The present application has Government rights assigned to theEnvironmental Protection Agency under Contract Number R82-6769-010.

BACKGROUND OF THE INVENTION

[0002] A. Field of the Invention

[0003] The present invention relates generally to chemical analysis ofaerosols, and, more particularly to a system and method for chemicalanalysis of individual particles in a high velocity gas flow using laserablation ion mobility spectrometry.

[0004] B. Description of the Related Art

[0005] Time-of-flight mass spectrometry is a well-known technique forquickly and accurately providing ion mass information. Time-of-flightmass spectrometry systems accelerate ions, via an electric field, towarda field-free flight tube which terminates at an ion detector. Inaccordance with known time-of-flight mass spectrometry principles, ionflight time is a function of ion mass so that ions having less massarrive at the detector more quickly than those having greater mass. Ionmass can thus be computed from ion flight time through the instrument.

[0006] Another known ion separation technique which may be used toseparate the bulk of the ions in time is ion mobility spectrometry. Ionmobility spectrometry instruments typically include a pressurized staticbuffer gas contained in a drift tube which defines a constant electricfield from one end of the tube to the other. Gaseous ions entering theconstant electric field area are accelerated thereby and experiencerepeated collisions with the buffer gas molecules as they travel throughthe drift tube. As a result of the repeated accelerations andcollisions, each of the gaseous ions achieves a constant velocitythrough the drift tube. The ratio of ion velocity to the magnitude ofthe electric field defines an ion's mobility, wherein the mobility ofany given ion through a high pressure buffer gas is a function of thecollision cross-section of the ion with the buffer gas and the charge ofthe ion.

[0007] Time-of-flight mass spectrometry has the ability tosimultaneously analyze all ions from each particle. This capability isalso shared by other known mass spectrometry methods such as Fouriertransform ion cyclotron resonance and quadruple ion trap. Thedisadvantages of these techniques is the need to operate under highvacuum conditions which adds complexity, size, and cost to the testinstrument. Ion mobility spectrometry overcomes these limitations bepermitting ion analysis to be performed at a pressure close toatmospheric pressure. Ion mobility spectrometry also retains the abilityto simultaneously analyze all ions from each particle. However, there isa need in the art to analyze individual particles in a high velocity gasflow.

SUMMARY OF THE INVENTION

[0008] The present invention satisfies this need by providing a systemand method for chemical analysis of individual particles in a highvelocity gas flow. The present invention further provides an ionmobility spectrometry system and method that analyzes individualparticles in a high velocity gas flow.

[0009] Additional objects and advantages of the invention will be setforth in part in the description which follows, and in part will belearned from the description, or may be learned by practice of theinvention. The objects and advantages of the invention will be realizedand attained by means of the and combinations particularly pointed outin the appended claims.

[0010] To achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, the inventioncomprises a system for chemical analysis of single particles in a highvelocity gas flow, the system comprising: an ion source chamber; a laserfor ablating the single particles in the high velocity gas flow enteringthe ion source chamber to produce positively- and negatively-chargedions from each single particle; means for extracting thepositively-charged ions provided in the ion source chamber; means forextracting the negatively-charged ions provided in the ion sourcechamber; a first ion mobility spectrometer connected to thepositively-charged ion extracting means and characterizing and detectingthe positively-charged ions; and a second ion mobility spectrometerconnected to the negatively-charged ion extracting means andcharacterizing and detecting the negatively-charged ions.

[0011] To further achieve the objects and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention comprises a method for chemical analysis of single particlesin a high velocity gas flow, the method comprising the steps of:introducing the gas into an ion source chamber; ablating the singleparticles in the high velocity gas flow entering the ion source chamberwith a laser to produce positively- and negatively-charged ions fromeach single particle; extracting the positively-charged ions from theion source chamber; extracting the negatively-charged ions from the ionsource chamber; characterizing and detecting the positively-charged ionswith a first ion mobility spectrometer; and characterizing and detectingthe negatively-charged ions with a second ion mobility spectrometer.

[0012] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate one embodiment ofthe invention and together with the description, serve to explain theprinciples of the invention. In the drawings:

[0014]FIG. 1 is a schematic diagram of an ion source chamber used in thesystem of the present invention;

[0015]FIG. 2 is a schematic diagram of the overall system of the presentinvention, including the dual ion mobility spectrometers of the presentinvention; and

[0016] FIGS. 3(a)-3(d) show timing diagrams for a laser pulse,extraction plates, and an ion shutter used in the system of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017] Reference will now be made in detail to the present preferredembodiment of the invention, an example of which is illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

[0018] In accordance with the invention, the present invention isbroadly drawn to a system and method for real-time chemical analysis ofsingle particles in a high velocity gas flow using laser ablation anddual ion mobility spectrometers. As used herein, the term “high velocitygas flow” means a gas flow traveling at a velocity up to 400meters/second (m/s), with particles traveling at a velocity of up to 150m/s.

[0019] The overall system of the present invention is shown in FIG. 2,and includes the ion source chamber 10 shown in FIG. 1. As embodiedherein and as shown in FIG. 1, a gas enters ion source chamber 10 at ahigh velocity through a gas inlet 12. Individual particles in the gasentering ion source chamber 10 are ablated with a laser 16 to producepositively- and negatively-charged ions from each gas particle.Preferably, laser 16 is a high-energy, pulsed, ultraviolet laser. Anygas remaining after the laser ablation exits ion source chamber 10through a gas outlet 14. A pair of ion extraction plates 18, 20 lieadjacent, and, preferably, orthogonal to the gas flow to extract ionstherefrom. Voltage pulses, preferably high voltage pulses, are appliedto extraction plates 18, 20 to establish a high electric field gradienttherebetween. The voltage pulses applied to extraction plates 18, 20 mayvary, however, applying a potential difference of 5000 volts (i.e.,applying a +2500 volt pulse to one plate, and applying a −2500 voltpulse to the other plate) between plates 18, 20 works quite well. Theelectric field gradient extracts ions from the gas flow throughapertures 19, 21 in the plates 18, 20. Positively- andnegatively-charged ions are extracted in opposite directions, withnegatively-charged ions 22 being extracted by plate 18 andpositively-charged ions 24 being extracted by plate 20. Alternatively,extraction plate 18 could extract positively-charged ions 24, andextraction plate 20 could extract negatively-charged ions 22. The typeof ions extracted by extraction plates 18, 20 depends upon the polarityof the voltage pulses applies to plates 18, 20 (e.g., ±2500 volts),wherein a negative voltage pulse applied to an extraction plate willattract positively-charged ions, and a positive voltage pulse applied toan extraction plate will attract negatively-charged ions.

[0020] As shown in FIG. 2, the negatively-charged ions 22 are injectedinto an ion mobility spectrometer 30. Ion mobility spectrometer 30includes an ion shutter 32 that receives a voltage pulse at a certaintime to maximize the resolution of the mobility analysis, a drift tube34 having a plurality of plates 36 with apertures provided therein, anda Faraday cup 38. Although a plurality of plates are shown in FIG. 2,drift tube 34 may also have only a single plate 36. After passingthrough aperture 19 (shown in FIG. 1) of extraction plate 18, thenegatively-charged ions 22 become trapped in ion shutter 32.Subsequently, the voltage applied to ion shutter 32 is pulsed to a newvalue to allow the trapped negatively-charged ions 22 to enter intodrift tube 34 and eventually strike Faraday cup 38. This allows thenegatively-charged ions 22 to be characterized and detected.

[0021] Similarly, the positively-charged ions 24 are injected intoanother ion mobility spectrometer 40. Like spectrometer 30, ion mobilityspectrometer 40 includes an ion shutter 42 that receives a voltage pulseat a certain time to maximize the resolution of the mobility analysis, adrift tube 44 having a plurality of plates 46 with apertures providedtherein, and a Faraday cup 48. Upon entering aperture 21 (shown inFIG. 1) of extraction plate 20, the positively-charged ions 24 becometrapped. Subsequently, the voltage applied to extraction plate 20 ispulsed to a new value to allow the trapped positively-charged ions 24 toenter into ion shutter 42 and drift tube 44 and eventually strikeFaraday cup 48. This allows the positively-charged ions 24 to becharacterized and detected.

[0022] Although there is no preferred voltage applied to ion shutters32, 42, the voltage applied to these shutters 32, 42 needs to be largeenough so that shutters 32, 42 may act like shutters, that is, eitherallow ions to pass through, or prevent ions from passing through.

[0023] As shown in FIG. 2, laser 16, extraction plates 18, 20, ionshutters 32, 42, and Faraday cups 38, 48 are all interconnected to acontrol unit 100, which may be any conventional controller, such as aprogrammable logic controller (PLC), a general purpose personal computerprogrammed with control software, etc. Control unit 100 determines whenlaser 16, extraction plates 18, 20, and ion shutters 32, 42 are to bepulsed, records the arrival time of the ions at Faraday cups 38, 48,and, according to conventional ion mobility techniques, as described inGary A. Eiceman and Zeev Carpas, Ion Mobility Spectrometry (CRC Press,Boca Raton, Fla. 1994), characterizes the ions by the elapsed or drifttime thereof. Thus, the size and chemical composition of individualparticles in a high velocity gas flow may be determined. FIGS. 3(a)-3(d)show the timing diagrams for when laser 16 is pulsed, when extractionplates 18, 20 are pulsed, and when ion shutters 32, 42 are pulsed,respectively.

[0024] The dual ion mobility spectrometer configuration of the presentinvention allows characterization of both the positively- andnegatively-charged ions from a single particle in a high velocity gasflow. The present invention may be used to provide the size and chemicalcomposition of ambient air particles, making the invention useful inpollution monitoring, industrial hygiene, and atmospheric chemistrystudies.

[0025] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A system for chemical analysis of singleparticles in a high velocity gas flow, the system comprising: an ionsource chamber; a laser for ablating the single particles in the highvelocity gas flow entering the ion source chamber to produce positively-and negatively-charged ions from each single particle; means forextracting the positively-charged ions provided in the ion sourcechamber; means for extracting the negatively-charged ions provided inthe ion source chamber; a first ion mobility spectrometer connected tothe positively-charged ion extracting means and characterizing anddetecting the positively-charged ions; and a second ion mobilityspectrometer connected to the negatively-charged ion extracting meansand characterizing and detecting the negatively-charged ions.
 2. Asystem as recited in claim 1, wherein the means for extracting thepositively-charged ions comprises an ion extraction plate arrangedadjacent and orthogonal to the high velocity gas flow.
 3. A system asrecited in claim 1, wherein the means for extracting thenegatively-charged ions comprises an ion extraction plate arrangedadjacent and orthogonal to the high velocity gas flow.
 4. A system asrecited in claim 1, further comprising: a control unit connected to thelaser, the means for extracting the positively-charged ions, the meansfor extracting the negatively-charged ions, and the first and second ionmobility spectrometers, the control unit determining the size andchemical composition of the single particles based upon thepositively-charged and negatively-charged ions characterized anddetected in the first and second ion mobility spectrometers.
 5. A systemas recited in claim 1, wherein the laser comprises a high-energy,pulsed, ultraviolet laser.
 6. A system for chemical analysis of singleparticles in a high velocity gas flow, the system comprising: an ionsource chamber; a laser for ablating the single particles in the highvelocity gas flow entering the ion source chamber to produce positively-and negatively-charged ions from each single particle; a first ionextraction plate arranged adjacent and orthogonal to the high velocitygas flow for extracting the positively-charged ions; a second ionextraction plate arranged adjacent and orthogonal to the high velocitygas flow for extracting the negatively-charged ions; a first ionmobility spectrometer connected to the positively-charged ion extractingmeans and characterizing and detecting the positively-charged ions; asecond ion mobility spectrometer connected to the negatively-charged ionextracting means and characterizing and detecting the negatively-chargedions; and a control unit connected to the laser, the first and secondion extraction plates, and the first and second ion mobilityspectrometers, the control unit determining the size and chemicalcomposition of the single particles based upon the positively-chargedand negatively-charged ions characterized and detected in the first andsecond ion mobility spectrometers.
 7. A system as recited in claim 6,wherein the laser comprises a high-energy, pulsed, ultraviolet laser. 8.A method for chemical analysis of single particles in a high velocitygas flow, the method comprising the steps of: introducing the gas intoan ion source chamber; ablating the single particles in the highvelocity gas flow entering the ion source chamber with a laser toproduce positively- and negatively-charged ions from each singleparticle; extracting the positively-charged ions from the ion sourcechamber; extracting the negatively-charged ions from the ion sourcechamber; characterizing and detecting the positively-charged ions with afirst ion mobility spectrometer; and characterizing and detecting thenegatively-charged ions with a second ion mobility spectrometer.
 9. Amethod as recited in claim 8, wherein the positively-charged ions areextracted by an ion extraction plate.
 10. A method as recited in claim9, wherein the negatively-charged ions are extracted by another ionextraction plate.
 11. A method as recited in claim 10, furthercomprising: connecting a control unit to the laser, the ion extractionplates, and the first and second ion mobility spectrometers; anddetermining, with the control unit, the size and chemical composition ofthe single particles based upon the positively-charged andnegatively-charged ions characterized and detected in the first andsecond ion mobility spectrometers.